Showing voltage over disconnect breaker when off?

10 Posts
Oct 2, 2009 08:34 am
Showing voltage over disconnect breaker when off?

Shouldn't I be showing an "open" when measuring the voltage when the disconnect breaker is off?   

I have the disconnect breaker wired on the top going to the positive inverter connection and on the bottom to the positive battery bank terminal and the positive pv output breaker.  When I measure the voltage across the breaker and negative post for the battery connection I'm getting a voltage. I'm taking the reading at the bottom of the disconnect breaker (bottom post on the breaker to the negative terminal on the negative post in the box where the negative battery connection hooks up) I see the voltage of the battery bank.  Why am I not showing an open?
Thanks in advance.
Ann
« Last Edit: Oct 2, 2009 08:46 am by Ann Byers »
 
578 Posts
Oct 2, 2009 10:25 am
Re: Showing voltage over disconnect breaker when off?

good question, lets go socratic method.

if you reply, tell us which load center is in use, and i can provide more detail.

"I have the disconnect breaker wired on the top going to the positive inverter connection and on the bottom to the positive battery bank terminal and the positive pv output breaker"

Q: so, where is the bottom of the breaker connected to?
A: "and on the bottom to the positive battery bank terminal"

Q: so . . . if the bottom of the inverter disconnect circuit breaker is connected to the positive terminal on the battery bank can it ever be off?

A: no, because it is opening the circuit between the battery terminal and the inverter, not the battery and the box, or more specifically the bottom of the breaker.

Q: does the same hold true for the negative busbar?

A: yep, the busbars (and bottom of inverter disconnect either holding one or acting like one) are extensions of the battery bank terminals.  they are always live, as you cant turn the batteries off.

because the busbars in a DC load center are always energized by the battery bank, it is very important that safety gear (appropriate ppe) and precautions are used when operating inside one.  the disconnects (pv, controller, inverter, loads) all
disconnect each other from sources.  with pv and batteries, it is important to remember that opening a disconnect may stop current flow after that disconnect, but there is no stopping the potential for voltage (and current in the event of a short) from a source before that disconnect. 

you would expect voltage across the postive and negative busbars in a dc load center.  they are merely an extension of the terminals inside a box, to facilitate connecting all those disconnects and branch circuits.  should you try to measure current (WHICH YOU SHOULD NOT) you may have an explosion as you would short the battery bank.

hope that helps,

james
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184 Posts
Oct 2, 2009 12:03 pm
Re: Showing voltage over disconnect breaker when off?

Ann,

You said:

I have the disconnect breaker wired...on the bottom to the positive battery bank terminal...

And:

I am taking the reading at the bottom of the disconnect breaker...and the negative post (of the battery)...

Therefore:

You have the meter connected across the battery.  In this case you would measure the full battery voltage.

John
 
10 Posts
Oct 2, 2009 12:12 pm
Re: Showing voltage over disconnect breaker when off?

James, thanks very much for your quick response. Somehow I was expecting an open but I"m glad to know it's wired OK..I just somehow expected the breaker to create open the circuit.  I'll disconnect the terminal to one battery post before I do any more work inside the box.  I guess I'll have to re-read what the DC quick disconnect actually disconnects.

TO John:  I guess I was not clear in my post.  I was not taking a voltage from the bottom of the breaker to a the battery terminal I was taking the voltage from the bottom of the DC disconnect breaker to the negative post on the **Inside Negative Post*** in the DC box...I guess it's considered the negative bus bar.   I don't think the leads on my multimeter would reach from the inside of the DC positive breaker to the battery bank Wink Wink Smiley

Thanks again everyone!!!
 
578 Posts
Oct 2, 2009 12:34 pm
Re: Showing voltage over disconnect breaker when off?

i hesitate to speak for john here, but i believe what he meant by saying
 "you have the meter connected across the battery"

is that by connecting your meter to terminals connected to the battery pos and neg busbars, essentially and electrically it is the same thing as measuring voltage across the battery bank posts themselves.  the only difference is the addition of the resistance of the conductors, which for your purposes is irrelevant.

notes for caution!!  rather than disconnect the lead from the dc enclosure with it still attached to the battery post, if you have any series connections in your battery bank, i would remove those and leave the other connections put. 

to put it another way, remove one series link from each series string you have, while leaving the connections in the dc load center alone.  then measure voltage at the terminals in the dc load center and in the battery bank where the conductors go from the battery posts to the load center.  since you would have removed one series connection from each string, you should have an open circuit both in the load center and in the battery box.  of course when you do this, have your appropriate ppe, and make sure your meter is set to dc voltage.

when you are all done with your wiring, while leaving all disconnects off, then replace the series connections (with your ppe on).

for what it is worth, i would suggest bringing in a pro if at all possible.  its not that i mind at all answering the questions, but batteries (and most electricity for that matter) can kill you with 1 mistake, and the question while good for theory, gives me some pause to know that it represents the lack of understanding that may put you in danger.  be careful

james
altE staff

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854
 
10 Posts
Oct 2, 2009 01:32 pm
Re: Showing voltage over disconnect breaker when off?

James,
Thank you again.
I wish I could find a willing master electrician or PV installer to help me but I have not been able to find anyone willing to even give me a call back. I have the entire system all wired (hopefully correctly) again I wish there was even a willing master electrician that I could hire but there are no takers. 
I appreciate all your help I guess I should stop at this point and just hope that at some point I'll find an electrian or PV installer who will check my work.  (all I have left is to snap the solarline connectors together and the system will be energized..if my wiring in the box to the solar controller and battery bank and inverter is correct).
Quote by James:
"rather than disconnect the lead from the dc enclosure with it still attached to the battery post, if you have any series connections in your battery bank, i would remove those and leave the other connections put."

Yes, I want you to know I was not disconnecting in the dc box.  I have the batteries wired in paralell and I disconnected the first lead to the negative side in the battery bank.


Thanks, I will take your advice and maybe sometime within my lifetime there will be some professional who will be willing to help  me....I will stop working on the system..and I'm incredibly disappointed.
Ann
« Last Edit: Oct 2, 2009 02:37 pm by Ann Byers »
 
4 Posts
Oct 2, 2009 03:09 pm
Re: Showing voltage over disconnect breaker when off?

Don't give up. post some pictures. Worth a thousand words.  that way people can look over your wiring and spot anything amiss

something like this mess
http://www.energyallover.com/installed_an.html

Now everyone can see and tell me I should have used a combiner box instead of double tapping the charge controller..

Robbie Dinion
« Last Edit: Oct 2, 2009 03:14 pm by Robert Dinion »
 
220 Posts
Oct 2, 2009 08:37 pm
Re: Showing voltage over disconnect breaker when off?


 hi ann,

 i'm with robbie on this one...you can't put those wrenches down yet! sounds like you're all set to go and just need a once over to be sure.

 i would recommend that you stop back at the radio club. these guys & gals from the league work with some good size power supplies and switch gear every day. many of the folks involved in amateur radio may have limited formal schooling but can hold their own on the levels of EE's & PE's

 we get folks stopping by all the time and there is always someone eager to help. they may come off as somewhat of "odd ducks" at times but are some of the brightest too.

 any folks out there looking for sound electrical advice might consider stopping by the local "ham" radio club. they are located across the world. your town hall or the ARRL in newington ct. would know the contacts as they are a big part of the civil defense system.

 kind regards, dave
 73 de kb1mzf
 
Oct 3, 2009 07:33 am
Re: Showing voltage over disconnect breaker when off?

Wow! Reading some of these posts I began to realize that "talking" and "writing" electrical can become a language all on its own. The English language may not be totally adaptable to electrical language. Kinda like if someone were to read musical notation over a telephone to someone else. For example, A minor, C, A minor, B, but, one can't here the music over the phone. You would need a musician with a musical instrument. But you know this. You know you need an electrician or at least would like to have an electrician check your work.
Even among electrical workers there can be miscommunication. There have been several occurrences here at this forum. For example, I mentioned in one that #12 awg copper is good up to 20 amps. Which is primarily true but depending on the circumstances under which it is used it may only be good for 5 amps or less. But it doesn't stop there. There are so many different fittings and conduits and conduletts and all in many different sizes, it can get very frustrating. Even an electrical schematic, a lot of the time, is a typical rendering and may leave one with questions concerning a specific problem to overcome on a particular adaptation. I had a master electrician tell me that it was code that utilities power must go on the top lugs of a transfer switch and the back up power on the lower lugs with of course the load on the middle lugs. Horse pucky! There is no such code in the NEC. It maybe the most popular concensus among licensed electrical contractors and electrical inspectors, but its not a code.
Now we start to get into the nitty gritty of it all and thats if, you could put it all into a pot and boil it down to its trace minerals what you would be left with is, money! Or in some cases, a lack of money! Someone spends their time and money to gain an education in some genre or other because they want to use it to earn a substantial income. Ok, most people but not all.
The internet is changing that in its own way but that in its self still takes a certain amount of self education and hands on experience and even a little trail and error. The error typically meaning money spent to learn a lesson and as Jame's pointed out it could cost one the ultimate price where electrical is concerned. I mean, how do we know that the first person to invent gunpowder was really the first? The first one may have blown themselves up in the process with nobody the wiser for it. This is starting to get lengthy I know, so I'll move on.
I am sure you have read many books and magazines on the subject of electrical wiring but what you lack is confidence that what you know and what you have installed is correct. Am I right? Doubts pop up and you would like a second opinion. Your not alone Ann Byers. This happens to everybody in every occupation, even the learned ones. One thing that make electrical frustrating sometimes is the language. Phrases like LINE and LOAD are agreed upon. LINE is the power source and LOAD is the device using power. Although LINE typically goes in at the top of a breaker and LOAD out off the bottom, its not always the case. So its typically referred to as just LINE and LOAD.
You have full voltage on the LINE side of your breaker at all times, but only on the LOAD side when the breaker is in the CLOSED position. LOAD is lost when the breaker is in the OPEN position. LINE should be considered eternal even though we now its not. Makes no difference if its a battery, PV module or, 120/240 vac from a pole mounted utilities transformer. OK!
If I assume you meant to say that you were getting;
(A) some voltage - or
(B) full voltage  (which one?)
on the LOAD side of the breaker with the breaker in the OPEN position, would this be correct?
If (A) some voltage - then how much and what is the nominal voltage? You might be reading resistance from a device (load side) through the negative. Your meter could be acting like a partial conductor, almost but, not quite, completing the circuit just enough to give a reading on your meter.
If (B) full voltage - you might have a bad breaker or polarity is reversed somewhere.
 
10 Posts
Oct 3, 2009 01:40 pm
Re: Showing voltage over disconnect breaker when off?

Robbie and Dave thank you so very, very much.  Thank you both for picking me up, lifting my spirits.   I"m back on the project.   I did post photos on my blog today and I give thanks to both of you!!!

And Thomas your quote really hit home with me:

"Doubts pop up and you would like a second opinion. Your not alone Ann Byers. This happens to everybody in every occupation, even the learned ones."

I do have doubts and I don't want to be the person who blows up but there would never be progress if everyone in the world lived in a bubble.  My favorite quote is from Reinhold Messner; "I have no intention of perserving my body for a nursing home".

Minutes ago I flipped the breaker that runs to the battery bank and the Solar Controller powers up and sees the voltage on the battery bank.

Dave, by the way I am also a ham, K1QO I've done some wiring and have built a few small qrp radios that's where my limited DC wiring comes in but I've never done anything with this amount of voltage and current.  And you are right...hams are a wonderful resource I'll be seeing them again on Wednesday night at our club meeting.

 

http://harvestingthesun.blogspot.com/

Thanks so much everyone,
Ann
(K1QO)

 
220 Posts
Oct 3, 2009 06:32 pm
Re: Showing voltage over disconnect breaker when off?


 holy cats! a four letter extra!!!

 if the folks here only knew the rigorous testing and body of knowledge required for that fcc ticket.

 i think i see the uncertainty you were having after spending so much time in the qrp niche. (tiny ultra low power transceivers). you have been working in the micro world of power compared to this macro power setup here.

 so happy to hear you powered up and decided to complete the system. still might be a good idea to have one of the high power HF guys over to have a look as well if you still can't get a "real" expert in.

fun blog you have there "solargoddess"

good luck, YL!

OM, dave
 

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